1. Field of the Invention
This invention relates to a leading body for drilling the ground and also to a ground drilling machine. More particularly, the present invention relates to a leading body for drilling the ground and also to a ground drilling machine to be used for drilling holes under the ground for the purpose of laying gas pipes, electric power cables, sheaths pipes for signal cables, optical fiber cables, water supply pipes, drainage pipes and so on without openly excavating the ground and also for drilling holes in rock beds in order to set dynamite there for blasting.
2. Description of Related Art
In recent years, efforts have been paid to develop ground drilling machines for drilling holes under the ground for the purpose of laying various pipes and cables without openly excavating the ground.
FIGS. 9 and 10 of the accompanying drawings schematically illustrate such a ground drilling machine 1. The illustrated ground drilling machine 1 comprises a horizontal drill unit 2 and a digging liquid supply vehicle 3 for supplying digging liquid to the horizontal drill unit 2. Rods 10 are sequentially fed out from the horizontal drill unit 2 in such a way that each succeeding rod is linked to the immediately preceding rod. A pilot head (drill) 20, which is a leading body, is rotatably fitted to the front end of the leading rod 10.
The ground drilling machine 1 is used with a horizontal drilling technique in order to lay pipes under the ground.
Firstly, the pilot head 20 is driven into the ground from en entrance pit A at a predetermined angle of inclination (10 to 20°). Then, after correcting the direction in which the pilot head 20 is propelled to a horizontal direction, the pilot head 20 is driven to rotate and dig the ground until it gets to starting pit B. Thereafter, the pilot head 20 is driven to rotate and dig a hole from the starting pit B to the destination pit C so as to produce a leading hole D. Between the starting pit B and the destination pit C, the position, the depth from the surface, the inclination, the rotary angle and so on of the pilot head 20 are detected by means of a magnetism detector located on the ground that detects the magnetic field produced by the transmitter (sonde) contained in the pilot head 20 so as to appropriately correct the direction in which the pilot head 20 is propelled while it is digging the leading hole D. During the digging operation, digging liquid that may be clean water, muddy water or bentonite solution is supplied from the digging liquid supply vehicle 3 and through the rods 10 and injected from the pilot head 20.
After digging the leading hole D, the pilot head 20 fitted to the front end is replaced by a reamer 30 for broadening the leading hole and the object of underground placement E that may be a pipe or a cable to be laid is connected to the reamer 30 by way of a swivel joint. Then, the reamer 30 is rotated back, while causing the reamer 30 to eject digging liquid, and the object of underground placement E is drawn into the hole, while broadening the leading hole D by means of the reamer 30.
During the above described operation of laying pipes under the ground, digging solution is injected from the pilot head 20 substantially in the direction of propelling the pilot head 20 in order to improve the efficiency of digging the leading hole D. The ground is loosened due to the injected liquid to facilitate the digging operation. Additionally, the injected digging solution is drained toward the starting pit B through the gap between the leading hole D and the rods 10 along with the dug soil. In other words, the injected digging solution also serves to deliver the dug soil.
However, the dug soil cannot be satisfactorily removed simply by means of the flow of digging solution when the digging efficiency of the ground digging machine is improved because a large volume of soil is dug in a very short period of time.
Additionally, if the dug soil is not removed efficiently, some of the dug soil remains near and in front of the pilot head 20 so that, if the revolving pilot head 20 is propelled further, the remaining dug soil is dug again to reduce the efficiency of digging the ground ahead of the pilot head 20. Furthermore, some of the dug soil can remain between the leading hole D and the pilot head 20 and/or between the leading hole D and the rods 10. Then, the load relative to the digging torque is reduced to further increase the digging efficiency.